A 1:1 protection scheme is standardized by ITU-T (International Telecommunication Union) (ITU-T Recommendation G8031). In this scheme, a transmission device and a reception device are connected via two lines in order to improve the reliability of communication, and in the initial state, communication is performed using one of the lines, while at the time of maintenance work or failure, communication is performed using the other line. In particular, in the 1:1 protection standardized by the ITU-T, a line can be switched to another line by the unit of VLAN traffic based on a switching request by a user.
In the 1:1 protection standardized by the ITU-T Recommendation G.8031, the line switching sections of a transmission device and a reception device are synchronized with each other to switch a line. Meanwhile, another scheme is also proposed, in which a reception device regularly receives frames from the both lines, and 1:1 protection is realized by only switching the line of the transmission device side (see Patent Document 1, for example).
It should be noted that in this description, a frame of the Ethernet (registered trademark) is simply called a frame or a packet.
Patent-Document 1: JP 10-145374 A
In the 1:1 protection standardized by the ITU-T, if the switching target line is a high-speed line having a smaller delay quantity compared with that of the switching source line, there is a problem that a packet loss is caused when switching lines. For example, when a transmission device transmits a packet 1, a packet 2, a packet 3, and a packet 4 in this order to the switching source line, and then transmits subsequent packets, namely, a packet 5, a packet 6, a packet 7, and a packet 8, in this order to the switching target line, if the switching target line is a higher-speed line compared with the switching source line, the packets 5 and 6 reach the reception device before the packets 3 and 4 reach the device, as shown in FIG. 6. As such, if line switching is performed immediately after the packet 2 is received from the switching source line, the packets 3 and 4 will be lost. On the contrary, if line switching is performed immediately after the packet 4 is received from the switching source line, the packets 5 and 6 will be lost.
On the other hand, in the scheme described in Patent-Document 1, as the reception device is able to receive not only the packets 1 to 4 but also the packets 5 to 8, a packet loss can be prevented. However, a state where the packets 5 and 6 are received before the packets 3 and 4 may be caused. As such, there is a problem that the sequence of the packets will be changed.